Background of the Invention
Field of the Invention:
[0001] This invention relates to a circuit for sensing the change in capacitance of variable
capacitor and, more particularly, to a sensor circuit for detecting excessive amounts
of water in the fuel tank of a vehicle by sensing the capacitance change of a probe
capacitor positioned within the fuel tank.
Description of the Prior Art:
[0002] In attempt to conserve the earth's limited reserve of fossil fuel, many automotive
manufacturers are increasingly building more and more automobiles that are driven
by diesel fuel supplied internal combustion engines. Diesel engines are generally
more fuel efficient than relatively sized gasoline operated engines. A problem related
more to diesel engines arises if water is allowed to accumulate in the diesel fuel.
Due to the nature of the diesel engine, water in the fuel can damage or even destroy
the fuel pump, fuel system and or fuel injectors of the diesel engine.
[0003] Until recently diesel engines were mainly used only in commercial vehicles such as
earth moving equipment, buses, etc. and very expensive automobiles. Because of the
expensive nature of these vehicles, complex and expensive filter systems could be
utilized to inhibit water from getting into the diesel fuel. However, as diesel engines
are now being used in massed produced consumer oriented automobiles, the automotive
manufacturers are in need of a simple and reliable sensor circuit that can detect
excessive amounts of water in the fuel such that the operator can be alerted to this
condition.
[0004] Generally, it is desired to have a dash mounted panel light that can be lit up on
the dash panel of the vehicle to warn the operator of water in the fuel tank. The
operator would as soon as possible thereafter have the fuel tank drained to remove
the excess water.
[0005] It is generally known that water can be differentiated from the diesel fuel by its
dielectric constant. Since water does not homogeneously mix with the fuel but rather
goes to the bottom of the fuel tank, it's presence can be determined by utilizing
a capacitor which varies in value as water is displaced thereabout and comparing it's
value to a reference capacitor. In fact, at least one automobile manufacturer uses
such a scheme for detecting water in the fuel. Basically, this prior art scheme uses
an oscillator to regularly charge up both capacitors while allowing the capacitors
to discharge through fixed value resistors while at the same time comparing the voltages
developed there across. Water causes an increase in the capacitor size of the variable
capacitor and thus the voltage developed thereacross which eventually will cause the
output of the comparator to go positive at a predetermined point. This output is then
rectified and filtered to be used to turn on a warning lamp.
[0006] Several problems are associated with this prior art system. The system is relatively
expensive, utilizing building block integrated circuits in combination with discrete
devices. Additionally, the system suffers in its accuracy. Hence, a need exists for
such a system which is suitable to be manufactured in integrated circuit form to reduce
system cost while increasing system accuracy.
Summary of the Invention
[0007] Accordingly, it is an object of the present invention to provide an improved sensing
circuit for detecting variations in the capacitance of a capacitor.
[0008] Another object of the present invention is to provide an improved Water-In-Fuel Sensor
Circuit for detecting excessive water levels in fuel.
[0009] In accordance with the above and other objects there is provided an improved sensing
circuit for detecting the change in capacitance of a variable capacitor, which includes
a reference capacitor that is coupled essentially in parallel with the variable capacitor.
An oscillator is used to alternately charge and discharge both capacitors between
first and second voltage levels. The resulting current through the reference capacitor
varies in magnitude as the size of the variable capacitor varies. The absolute magnitude
of this current is detected and compared with a known current wherein the difference
current therebetween is indicative of the variations in the capacitor size of the
variable capacitor.
[0010] One feature of the present invention is that the sensing circuit is suited to be
fabricated in integrated circuit form to which the variable capacitor and reference
capacitor are then connected. The sensing circuit is utilized to detect excessive
water in the fuel of a vehicle. As the water content increases, the capacitance of
the variable capacitor increases thereby reducing the magnitude of the rectified current.
As the magnitude of the rectified current decreases below the magnitude of the known
current the sensing current produces a useful output signal that may be utilized to
turn on a warning lamp.
Brief Description of the Drawings
[0011]
FIG. 1 is a partial blocl; diagram in a schematic illustrating the sensing circuit
of the present invention;
FIGS. 2A and 2B are detailed schematics illustrating the sensing circuit of the preferred
embodiment of the present invention; and
FIG. 3 is a partial block and schematic diagram illustrating a particular embodiment
of the detector circuit of the present invention.
Detailed Description of the Preferred Embodiment
[0012] Turning now to FIG. 1, there is illustrated sensor circuit 10 of the present invention
in general block diagram form. Sensor circuit 10 detects changes in the capacitance
value of capacitor 12, which may be a transducer or probe type capacitor element for
instance. Thus, capacitor 12 could be utilized to provide an indication of the environment
in which it is placed. For example, capacitor 12 may be a probe type capacitor that
is situated in the fuel tank of an internal combustion engine driven vehicle to detect
the presence of water in the fuel. Such probes are available and have been used by
the automotive industry in the past wherein the capacitance of the probe increases
or decreases as the water content increases or decreases. Hence, if as shown in F
IG. 1, capacitor 12 is coupled to sensor circuit 10, a warning light 14, that is situated
on the dash panel of the vehicle, could be lit to indicate that a harmful amount of
water content is in the fuel as capacitor 12 increases in value above a predetermined
value. In response, the vehicle operator would be alerted to immediately having the
fuel tank drained which prevents excessive water in the fuel from damaging the fuel
pump and fuel injectors of a diesel type combustion engine for example.
[0013] The manner in which sensor circuit 10 detects the capacitance change in capacitor
12 is now briefly explained. A known current I is shared between probe capacitor 12
and a reference capacitor 16. Capacitor 12 is referenced to a ground reference potential
while capacitor 16 is referenced to a virtual ground potential through detector circuit
20 such that these two capacitors, which are commonly coupled at node 22, are essentially
placed in parallel configuration with respect to each other. The known current I is
either sourced to or from node 22 as controlled switch 24 is either connected to current
source 26 or 28 respectively. The charge on the capacitor 12 and capacitor 16 is limited
by the supply voltage V
CC so that the polarity of the current I is regularly reversed by the action of oscillator
30 controlling switch 24 in conjunction with receiving a feedback signal via lead
32 from node 22. Hence capacitor 12 and capacitor 16 are alternately charged and discharged
between first and second voltage levels. The value of the current IT that flows through
capacitor 12 is a function of the capacitance value of this capacitor and can be determined
from the known value of I and the current I
R which flows through fixed reference capacitor 16. Thus, IT is equal to:
and;
therefore;
[0014] As shown above, the value of the reference current I
R is proportional to the variation in capacitance C
12 of capacitor 12 wherein the capacitance C
16 of reference capacitor 16 is fixed.
[0015] Magnitude detector 20 includes a full wave rectifier circuit which rectifies the
current I
R. The rectified current, which is equal to the absolute magnitude of the current I
R, is supplied to a comparator circuit included within detector circuit 20. The rectified
current is then compared by the comparator circuit to a current of known and predetermined
value, xI, which may be, as indicated, proportional to the total current I. The value
of the proportionality factor, x, is fixed and is made equal to:
[0016] Too much water in the fuel is therefore indicated when the absolute magnitude of
the current IR becomes less than the value of xI which causes the output of the comparator
circuit to trip or change output level states. The changing of output state of the
comparator is utilized then to turn on lamp 14 to warn the operator of this condition.
In response, the operator would have the fuel drained from the fuel tank to prevent
damage to the fuel system of the vehicle.
[0017] Although not specifically shown, it is considered apparent that in a general application
variable capacitor 12 and reference capacitor 16 could be interchanged wherein the
magnitude of current through the variable capacitor is directly detected and compared
with the known current. The circuit would function as aforedescribed.
[0018] Although not necessary to the present invention as described above, sensor circuit
10 may include a delay filter 34 and an amplifying circuit comprising predriver amplifier
36 and output amplifier 38. A time delay through circuit 34 is made to be substantially
equal to one cycle of operation of sensor circuit 10 to thereby inhibit tripping of
the system due to transient signals caused by inherent differences between the magnitudes
of the charging and discharging currents I as generated from oscillator 30. Hence,
the sensor circuit operates off the larger magnitude of current I which is measured
continuously by detector circuit 20. Predriver amplifier 36 amplifies the output from
filter 34 to provide an input to amplifier 38 which in turn provides sufficient current
to turn on lamp 14 as current flows therethrough between battery supply 40 and ground
reference. Also included is latch/hysteresis circuit 42 which is coupled between predriver
36 and detector circuit 20.
[0019] As the amount of water in the fuel becomes excessive (IR<xI) the output of the comparator
circuit of detector circuit 20 is caused to change output states and delay filter
capacitor 44 is allowed to charge up from ground reference. Capacitor 44 continues
to charge until the voltage developed thereacross exceeds a predetermined value which
thereafter renders predriver 36 operative. Latch circuit 42 is rendered operative
in response to predriver 36 becoming operative to increase the current, xI, by a predetermined
value which introduces hysteresis into the circuit by increasing the trip point at
which the comparator circuit will change output states. Thus, sensor circuit 10 is
latched into an on condition to maintain the warning lamp 14 turned on.
[0020] A lamp check circuit 46 may also be provided that is rendered operative as ignition
switch 48 of the vehicle is closed to light up lamp 14 for a predetermined interval
whereby the operator knows that lamp 14 is functional. The lamp check circuit 46 remains
operative until capacitor 50 is charged to a predetermined voltage level which renders
the circuit nonoperative to then turn off lamp 14.
[0021] Turning now to FIG. 2 there is shown sensor circuit 10 in more detail. Circuit 10
of FIG. 2 is suited to be manufactured in integrated circuit form except for the three
external capacitors 12, 16 and 44. Power supply conductor 60 is connected at external
coupling pad 62 to a source of operating potential, V
CC, which is supplied to sensor circuit 10. The operating potential may be obtained
from the battery voltage as shown in FIG. 2B. A master bias circuit 64 is coupled
between supply conductor 60 and ground reference potential 66 to provide a master
bias voltage potential between terminal 68 and 70 for biasing the plurality of PNP
current sourcing transistor 72, 74, 76, 78 and 80. Master bias circuit 64 is a circuit
that may generally be well known to those skilled in the art. The current sourcing
transistors each have a respective degeneration resistor coupled in the emitter paths
thereof as understood and may be either single or multi-collector output devices as
shown.
[0022] Capacitor 12, which may be a probe type capacitor, is coupled between the output
of oscillator 30 (at external connecting pad 82) and ground reference potential 66.
In the preferred embodiment probe capacitor 12 would be situated in the fuel tank
of a vehicle. Oscillator 30 both sources current to and sinks current from pad 82
of substantially equal magnitudes which charges and discharges capacitor 12 and capacitor
16 between first and second voltage levels. Thus, assuming no charge across capacitor
12, transistors 84, 86 and 88 are in a non-conductive state. Hence, transistor 90
is conducting which places a bias potential at the connection of resistors 92 and
94, at node 96, which insures that transistor 84 is turned off. Similarly, with transistor
88 being off, a bias potential is developed at the interconnection of series connected
resistors 98 and 100 to maintain transistor 102 conductive. Because the emitter of
transistor 102 is connected via resistor 104 to the emitter of transistor 106 to be
returned to ground reference through resistor 108, a bias potential is set up on the
emitter of transistor 106 which reverse biases this transistor into a non-conducting
state. In this state of operation, a current of magnitude I is supplied from the collector
of current sourcing transistor
78 to node 110 directly to pad 82 for charging capacitors 12 and 16 which are connected
at this node. Capacitor 12 and 16 are caused to be charged by the current I until
such time that the voltage at pad 82, and hence at the base of transistor 84, becomes
sufficient to overcome the back bias voltage maintain at the emitter of this transistor
as supplied from node 96. When this occurs, transistor 84 is rendered conductive which
establishes a bias current through resistor 112. Thereafter, transistor 86 is rendered
conductive as base current is supplied through transistor 84. This action causes transistor
102 to become non-conductive as all available base drive current thereto is now sourced
through the collector-emitter path of transistor 88. As transistor 102 is turned off
the reverse bias voltage at the emitter transistor 106 is removed.
[0023] Transistors 118, 120 and 106 are connected in a well known current mirror configuration
such that the current of magnitude I supplied to the collector of transistor 118 from
current sourcing transistor 80 is mirrored in the collector of transistor 106. As
indicated, the emitter area of transistor 106 is made twice (2A) the size of the emitter
area of transistor 118 such that the collector current of transistor 106 is made equal
to 21. Because transistor 78 can only supply a current of magnitude I, transistor
106 causes capacitors 12 and 16 to be discharged at proportional rates to supply the
additional current I. Therefore, as was previously described, oscillator 30 alternately
sources a current I to and then from pad 82. Transistor 106 will remain conductive
until such time that the voltage level appearing at node 82 decreases to a first level
voltage which renders transistor 84 non-conductive to allow transistor 102 to become
conductive thereby repeating the aforedescribed cycle.
[0024] Reference capacitor 16 is connected between pad 82 and external pad 122 to the input
of magnitude detector circuit 20. Detector circuit 20 includes a full wave current
rectifier circuit comprising transistors 124, 126, 128, 130, 132 and 134. Diode connected
transistors 136 and 138 which are coupled between ground reference to the collector-emitter
path of transistor 76 set the bias to transistor 128 and thus to the input transistors
124 and 126. The full wave rectifier is known in the art such that the operation thereof
is only briefly described hereinafter.
[0025] As a current IR is sourced to the input of the full wave rectifier, to the bases
of transistors 124 and diode-connected transistor 126, the current at the collector
of transistor 124 will be equal to IR and is sourced through diode connected transistor
130. Transistors 130, 132 and 136 are connected as a typical current turn around mirror
circuit such that a current of magnitude 2IR is sourced to node 137 via lead 139.
Similarly, as a current IR is sourced from the full wave rectifier through output
pad 122, an equal magnitude current flows through the collector-emitter path of transistor
128 which also produces a current of 2IR to node 137. Thus, in response to the current
IR that is alternately sourced to and from pad 122 (as capacitor 16 is charged and
discharged) a rectified current of magnitude 2IR is continuously sourced to node 137.
The magnitude of 'the rectified current maybe made any value and may be equal to IR.
In the present embodiment the-magnitude of the rectified current was doubled with
respect to the current flowing through reference capacitor 16 in order to overcome
any parasitic capacitance which may appear at the collector of transistor 140 due
to the nature of the structure of this transistor in integrated circuit form. It should
be also noted that as transistors 124 and 126 have their respective emitters connected
to ground reference 66 reference capacitor 16 is placed essentially at a virtual ground
reference. Thus, the two capacitors 12 and 16 are placed in substantially parallel
configuration.
[0026] In the preferred embodiment, probe capacitor 12 is made such that it extends into
the fuel tank whereby it is normally surrounded by fuel. However, water, either by
fuel contamination or condensation, occurring in the fuel tank will go to the bottom
thereof to surround capacitor 12. The dielectric constant of water, being different
from the fuel, causes the effective capacitance of capacitor 12 to increase from its
nominal value: the greater the percentage of water in the fuel the greater is the
magnitude of the effective capacitance of the probe capacitor. This variation of the
magnitude of the effective capacitance C
12 with increasing percentages of water in the fuel can be empirically determined. Thus,
from equation 4, a value x can be predetermined which is indicative of an excessive
amount of water in the fuel and at which warning lamp 14 is to be turned on.
[0027] In view of the above, because the value of the current IR is proportional to the
capacitance C
12 (see equation 3) and the total current I, the magnitude of current 2IR supplied to
node 137 can be compared with a fixed reference current xI to turn on lamp 14 when
ever the value of the current 2IR becomes less than the current xI: the trip point
of the aforementioned comparator. The manner in which this comparison is carried out
is by sinking the current xI from node 137 through the collector-emitter path of transistor
140. Sufficient base current drive is supplied to the base of transistor 140 to cause
a collector current of magnitude xI to flow therethrough between node 137 and ground
reference via the collector-emitter path of this transistor. Base current drive to
transistor 140 is supplied via current sourcing transistors 74 and 76 which have respective
collectors connected to diode connected transistors 142, 144, and 146. As illustrated,
diode-connected transistors 142, and 144 have there emitters commonly connected to
the base-collector electrodes of transistor 146, the emitter of which is connected
to the base of transistor 140, thereby forming a current mirror circuit.
[0028] In normal operation, with no water or with insufficient amounts of water in the fuel
to be-of concern, the value of the current 2IR sourced to node 137 exceeds the value
of the current xI. Transistor 148, which has its collector coupled via resistor 150
to power supply conductor 60, acts as the output of the comparator circuit and sources
current to the base of transistor 152: the base of transistor 152 being coupled to
the emitter of transistor 148. If, however, excessive water appears in the fuel the
value of the current 2IR at node 137 becomes less than the current xI that transistor
140 wants to sink. This state will cause transistor 148 to become nonconductive as
there is no longer sufficient base current drive available thereto. Thus, a current
proportional to the current flowing through capactor 12, which itself is proportional
to the magnitude of current flowing through probe capacitor 12, is compared with a
known current (xI) to cause switching of the output of the comparator transistor 148
whenever an excessive amount of water occurs in the fuel of the vehicle.
[0029] The input of delay filter circuit 34 is taken at the base of transistor 152. A capacitor
154 is coupled between the base and collector of transistor 152 which produces an
effective capacitance at the emitter of transistor 148 to ground reference that is
equal to the beta amplification factor of transistor 152 times the value of the capacitor
154. This effective capacitor filters switching transient signals that otherwise could
occur as transistor 148 is switched between a conducting and non-conducting state.
Transistor 152 and capacitor 154 act as an active filter having a delay period greater
than one cycle of operation to thereby inhibit false switching at the trip point which
might otherwise occur because the value of the current
I that charges capacitors 12 and 16 is not equal to the current I that discharges these
same capacitors. By providing filtering, the changing of states of transistor 148
is dependent only on the relative size of capacitor 12 to capacitor 16 and is not
dependent on any differences in the magnitude of the charging and discharging currents
supplied to or from pad 82.
[0030] As aforementioned, with little or no water in the fuel, transistors 148 and 152 are
conductive: collector current to transistor 152 being provided by current sourcing
transistor 72 having a respective collector coupled to the collector of transistor
152. Base current drive to transistor 156 is therefore inhibited as transistor 152
sinks all the current from the collector of transistor 72 connected thereto to ground
via the collector-emitter path thereof. Hence, transistor 156,. which has its collector
and emitter connected with in parallel with the collector and emitter of transistor
158 is turned off. Transistor 158 which is connected as a well known current mirror
circuit in combination with transistors 160, 162, and resistors 164, 166, and 168,
is turned on as transistor 156 is turned off to cause transistors 160, and 162 to
become conductive whereby all of the current source from multiple-collector transistor
72 to these transistors is shunted to ground reference. Therefor, transistor 162 holds
the plate of capacitor 44 that is coupled at pad 170 essentially at ground reference
to maintain transistor 172 in an non-conductive state.
[0031] Transistor 172, which forms with transistor 174 the predriver circuit 36 and which
in combination the aforementioned current mirror circuit may be considered an output
circuit, inhibits conduction of transistor 174 as no base current drive is supplied
thereto. Amplifier 38 which has its input coupled to the collector of transistor 174
is therefor maintained in an non-conductive state. Hence, with no water in the fuel,
lamp 14 remains off. However, as transistor 148 is turned off due to excessive water
in the fuel, transistor 156 will be turned on to turn off transistor 158 of the aforementioned
current mirror. Consequently, transistor 162 is also turned off. This allows capacitor
44 to begin charging towards V
CC.
[0032] When the voltage across capacitor 44 exceeds the turn on voltage of transistor 172
this transistor is rendered conductive to source current through is collector-emitter
path to node 178. Transistor 174 which has its base coupled to node 178 through resistor
180 is therefore turned on to then turn on amplifier 38. This causes lamp 14 to be
lit.
[0033] Prior to lamp 14 being turned on, in response to transistor 172 being rendered conductive,
transistor 182 is rendered conductive by transistor 172 since the base of this transistor
is coupled to node 178 via resistor 184. Resistor 186 is coupled between the emitter
of transistor 172 and ground reference for biasing purposes. Thus, base current drive
is stolen from respective transistors 188 and 190, each having the base thereof coupled
to the collector of transistor 182 via resistors 192 and 194 respectively. Therefore,
as all of the available current from power supply conductor 60 via resistor 196 is
now sourced through the collector-emitter path of transistor 182 these two transistors
are turned off. As transistors 188 and 190 are rendered non-conductive, more current
is allowed to be supplied via diode-connected transistors 142 and 144 to transistor
146. This in turn increases the base current supplied to transistor 140 to provide
a hysteresis action whereby the trip point at which transistor 148 is to be shat off
is raised high enough to latch sensor circuit 10 in' a conturing state that after
once setising excessive water in the fuel will cause lamp 14 to remain on until the
water is removed. In fact, the trip point is sufficiently high so that evea it the
selective value of capacitor C
12 is reduced to zero, the system will riot revert back to it's normal operating mode
until latch circuit 42 is disabled by a latch disabling signal supplied to pad 198.
[0034] If a latch disabling signal is applied to pad 198, transistor 200 is turned on as
it's base is coupled via resistors 202 and 204 to pad 198. As a transistor 200 is
rendered conductive, sufficient latch current is sinked to ground through the collector-emitter
path of transistor 200 that is coupled between the collector of transistor 188 and
ground reference to allow transistor 140 to be driven by a modified trip point base
drive current. This Introduces hysteresis into the circuit until such time that the
water level content is reduced to a minimum allowable percentage. Thereafter, transistor
190 would be rendered non-conductive. Hence, the collector current of transistor 140
is reduced to the original trip point value (xI). Now, because there is no longer
excessive water in the fuel, the value of current 2IR is sufficient to turn on transistor
148 and consequently to turn off transistor 172. Lamp 14 is then turned off and normal
operation of the sensor circuit is resumed. Zener diode 206 which is coupled between
the interconnection of resistors 202 and 204 and ground reference provides over voltage
protection to transistor 200. Resistor 208 is provided as a bias resistor and is coupled
between the emitter and the base of transistor 200.
[0035] Turning to FIG. 3, there is shown a modified portion of detector circuit 20 which
would allow detecting the currents through both capacitors 12 and 16 and sensing the
difference directly therebetween. In this embodiment, reference capacitor 16 is coupled
to a rectifier section 210 as previously discussed. Additionally, probe or vartable
capacitor 12 would have the electrede previously connected to ground reference connected
to the input of additional rectifier section 212. Rectifier section 212 is identical
to rectifier 210 and is shown in FIS. 2A comprising transistors 124, 126, 128, 136,
138 and current sourcing transistor 76. Current turn around and mirror circuit comprising
transistors 130, 132 and 134 could be replaced with current mirror circuit 214 of
similar construction wherein the current flowing from the input (indicated by the
half circle) thereof, due to the current flowing through capacitor 16, appears at
the output which is coupled to the base of transistor 148. This current is then directly
compared with the current flowing through capacitor 12 as the output of rectifier
212 is connected to the output of current mirror 214. The current sourced at the output
of rectifer 212 is directly proportional to the current flowing through capacitor
12. Thus, the difference current between the respective currents flowing through capacitors
12 and 16, which is proportional to the size of capacitor 16, can be used to switch
the operating state of comparator transistor 148 as described above.
[0036] Thus, what has been described above, is a novel sensor circuit which in the preferred
embodiment may by utilized for sensing excessive water in the fuel of a diesel fuel
operated internal combustion engine. The sensor circuit is suited to be fabricated
in integrated circuit form to reduce system cost while increasing the accuracy of
the system.
1. A monolithic integrated sensor circuit suitable for detecting a capacitance change
in a variable capacitor coupled therto, the sensor circuit being adapted to receive
an operating potential supplied thereto, comprising:
a first capacitor of predetermined value, said first capacitor being coupled to the
sensor circuit at first and second terminals thereof and being connected essentially
in parallel with the variable capacitor, the variable capacitor being connected at
one electrode to said first terminal of the sensing circuit, the other electrode of
which is referenced to a ground reference potential;
circuit means having an output coupled to said first terminal for alternately sourcing
a current to said first terminal and then sinking a current there from to charge and
discharge said first capacitor and the variable capacitor between first and second
voltage levels, the magnitude of current flowing through said first capacitor being
proportional to the capacitance value of the variable capacitor; and
detector circuit means having an input coupled to said second terminal for detecting
the absolute magnitude of said current flowing through said first capacitor and for
comparing said detected current to a current of predetermined magnitude wherein the
difference current therebetween is indicative of the capacitance value of the variable
capacitor.
2. The sensing circuit of claim 1 wherein said circuit means including an oscillator
circuit which is responsive to the voltage at said first terminal being less than
or equal to said first voltage level for sourcing a current, I, to said first terminal
and being responsive to the voltage at said first terminal rising to said second voltage
level for sinking a current, I, therefrom until the voltage at said first terminal
decreases to said first voltage level.
3. The sensing circuit of claim 2 wherein said detector circuit means includes:
a full wave rectifier circuit coupled to said second terminal for producing a rectified
current at an output thereof which is directly proportional to the magnitude of said
current flowing through said first capacitor;
current mirror means for establishing said current of predetermined magnitude, said
current mirror means having an output coupled with said output of said full wave rectifier
circuit wherein said rectified current is sourced by said current mirror means up
to the value of said predetermined magnitude; and
comparator means coupled to said outputs of said full wave rectifier circuit and said
current mirror means which is responsive to the value of said rectified current exceeding
the value of said current of predetermined magnitude for causing the output thereof
to switch from a first output level state to a second output level state.
4. The sensing circuit of claim 3 including:
filter means coupled to said output of said comparator means for inhibiting switching
transient signals whenever the output of said comparator switches btween said output
level states; and
output circuit means coupled with an output of said filter means which is responsive
to said output of said comparator switching to said second output level state for
producing an output signal a predetermined time interval thereafter.
5. The sensing circuit of claim 4 including:
latch circuit means coupled between said output of said output circuit means and said
current mirror means which is responsive to said ouput signal for causing the magnitude
of said current of predetermined magnitude to increase wherein the sensing circuit
is latched into its existing state of operation; and
disabling means responsive to a disable signal supplied thereto and which is operatively
coupled with said latch circuit means for disabling said latch circuit means wherein
said current established by said current mirror means is reduced to an intermediate
predetermined magnitude.
6. The sensing circuit of claim 5 including:
a lamp, having first and second electrodes, said first electrode being coupled to
the operating potential; and
amplifying means having an input coupled to said output of said output circuit means
and an output coupled to said second electrode of said lamp, said amplifying means
being responsive to said output signal for causing current to flow through said lamp
thereby lighting the same whenever the capacitance of the variable capacitor exceeds
a predetermined value.
7. In a vehicle having an internal combustion engine, a fuel tank and fuel supply
system for operating' the engine, a water-in-fuel sensor circuit, comprising:
a first capacitor disposed in said fuel tank wherein the capacitance value thereof
varies as water is displaced thereabouts, the capacitance being caused to vary from
a minimum to a maximum value in response to the level of water in the fuel tank increasing
from a nominal value to an excessive level;
a second capacitor of predetermined value, said second capacitor being connected essentially
in parallel to said first capacitor externally of the fuel tank;
circuit means for alternately charging and discharging said first and second capacitors
between first and second voltage levels, the magnitude of current flowing through
said second capacitor varying as said first capacitor varies in capacitive value;
and
detector circuit means for detecting the absolute magnitude of said current flowing
in said second capacitor and for providing an output signal whenever the magnitude
thereof decreases felow a predetermined value which is indicative of exessive water
levels in the fuel.
8. The sensor circuit of claim 7 wherein said circuit means is an oscillator for sourcing
a current, I, to said first and second capacitors and then sinking a current, I, therefrom
as said first and second capacitors are charged and discharged between said first
and second voltage levels respectively.
9. The sensor circuit of claim 8 wherein said detector circuit means includes:
a full wave rectifier circuit which is responsive to said current flowing through
said second capacitor for providing a rectified current at an output thereof that
is directly-proportional to the magnitude of said current;
current mirror means having an output coupled to said output of said full wave rectifier
cirucit for sinking a current of predetermined magnitude at said output thereof; and
comparator means having an input coupled to said outputs of said full wave rectifier
circuit and said current mirror means and having an output, said comparator means
being responsive to said magnitude of said rectified current becoming less than the
magnitude of said current of predetermined magnitude for causing the output thereof
to switch from a first output level to a second output level.
10. The sensor circuit of claim 9 including:
filter means coupled to the output of said comparator for inhibiting switching transient
signals; and
output circuit means coupled to an output of said filter means which is responsive
to said comparator output switching to said second output level for producing an output
signal therefrom which is indicative of an excessive amount of water content in the
fuel a predetermined time interval after the output of said comparator switches to
said second output level.